1. Field of the Invention
The present invention relates to a thermal printer, and more particularly to a thermal printer comprising a platen roller for advancing a transfer paper and an ink film in tight contact with each other in a longitudinal direction thereof. A thermal head for thermally transfers ink onto the transfer paper. The thermal head is movable between a transfer position to hold the transfer paper and the ink film in tight contact with each other between the thermal head and the platen roller and a non-transfer position to release the transfer paper and the ink film. A takeup roller takes up the ink film after the transfer.
2. Description of the Prior Art
In known thermal printers of this type, the platen roller is driven by a motor such as a pulse motor which is capable of stopping the roller at target positions with high precision to realize high precision for printing positions. The takeup roller requires less precision for its takeup action and is driven by an inexpensive motor such as a fixed speed motor. Therefore, a proper takeup action is carried out by driving the takeup roller drive motor when the thermal head is also in the transfer position. Relative rotation between the takeup roller and the takeup roller drive motor is permitted by a slip clutch.
In the prior art, a torque transmitted by the slip clutch has a fixed value as does a starting torque of the takeup roller drive motor. The transmitted torque and the starting torque are determined to enable the takeup roller to perform the takeup function as desired even in a heavy load condition in which a large amount of ink film is taken up.
With the above prior art arrangement, however, the ink film is subjected to a great deal of tension under a small load condition in which a small amount of ink film is taken up. This provides possibilities of trouble such as the ink film becoming torn. Because of the tension noted above, a separator roller for guiding the ink film away from the transfer paper tends to recede to a considerable extent to a position intermediate the ends of the ink film. As a result, the separation of the ink film takes place earlier at an intermediate position than at both ends. This impairs the quality of transfer at the intermediate position or in certain cases causes a total failure of the transfer. Such trouble may of course be checked by reducing a maximum amount of ink film takeup. However this would require a small roll of ink film which must be changed frequently. Therefore, this solution is hardly practicable.